The present invention relates to digital communications, and more particularly to a robust digital adaptive equalizer for use, e.g., in high-speed digital communications and digital television broadcasting, such as high definition television (HDTV).
In high-speed digital communication and digital television broadcasting (cable or wireless), digital adaptive equalizers are used to compensate for linear channel distortions. See, for example, Paik et al. U.S. Pat. No. 5,243,624 issued on Sep. 7, 1993 for xe2x80x9cMethod and Apparatus for Updating Coefficients in a Complex Adaptive Equalizerxe2x80x9d, incorporated herein by reference.
Linear channel distortions generate Inter-Symbol Interference (ISI). With ISI, a received symbol contains delayed or advanced adjacent symbols with modified amplitude and shifted phase. The decision-making device in a digital receiver (slicer) produces incorrect data output with severe ISI. The output from a modulator/transmitter contains a known amount of ISI when a square-root raised cosine filter is used. However, in an ideal channel, the receiver Nyquist filter removes this known amount of ISI. The slicer will reproduce the same output data as the input data at the modulator.
Multiple reflections and diffraction from man-made obstacles such as large buildings, or from terrain such as mountains or trees, create multi-path distortion of the transmitted signal. In open wireless channels, multi-path introduces ISI into the received signal. In cable-TV transmission, micro-reflections due to impedance mismatch from various passive or active elements such as taps, amplifiers, and coaxial cables also create ISI. In a modern digital receiver, ISI is removed by an adaptive equalizer. A review by Shahid U. H. Qureshi, xe2x80x9cAdaptive Equalizationxe2x80x9d, Proceedings of IEEE 73, 1349-1387 (1985) describes some of the commonly used adaptive equalizers. The design of update algorithms to speed-up the rate of convergence of adaptive equalizers has been a topic of intense study for more than two decades. The rate of convergence for conventional Least-Mean-Square (LMS) stochastic equalizers is very slow when noise and large ISI are present. The LMS stochastic equalizer may not be able to converge in the presence of severe noise and multipath echoes.
The present invention provides a new robust adaptive equalizer for a multiphase and/or multi-amplitude receiver such as a quadrature amplitude modulation (QAM) or vestigal side-band (VSB) receiver based on a modified computationally efficient LMS algorithm. This equalizer effectively removes the noise and ISI effect from the tap adaptation with fast and accurate adaptation of equalizer tap values. In fact, simulation results show that the LMS error magnitude converges more than 100 times faster than the conventional LMS algorithms. Simulation results further show that the inventive algorithm works equally well for signed, signed-signed and shift-and-signed stochastic LMS algorithms, which are commonly used in today""s high-speed digital receivers.
In accordance with the invention, a method is provided for updating coefficients (e.g., complex coefficients) in an adaptive equalizer. The equalizer has at least one equalizer filter stage with taps that receive the coefficients during successive filter clock cycles. A set of said taps that correspond to received echoes is identified. Only the taps in said set (i.e., those that correspond to received echoes) are adjusted according to a current error output from said equalizer. In this manner, the equalizer coefficients that do not correspond to received echoes remain fixed while the other equalizer coefficients that do correspond to received echoes are being adjusted.
The identifying step can use a trial-and-error method to identify the taps corresponding to the received echoes. Alternatively, the identifying step can use a sweeping method to identify the taps corresponding to the received echoes. In still a further embodiment, the identifying step can use an off-line processing method to identify the taps corresponding to received echoes. In an illustrated embodiment, the coefficients are updated using a moving window algorithm to enable different taps to be adjusted at different times.
An adaptive equalizer is provided which has at least one equalizer filter stage with taps that receive coefficients, such as complex coefficients, to be updated during successive filter clock cycles. Means are provided for selectively adjusting different ones of said taps in response to received echoes. More particularly, only taps corresponding to received echoes are adjusted, whereby the equalizer coefficients that do not correspond to received echoes remain fixed while the other equalizer coefficients that do correspond to received echoes are being adjusted.
In a more specific embodiment, an adaptive equalizer for a digital communications receiver is provided having at least one equalizer filter stage with taps that receive coefficients (e.g., complex coefficients) to be updated during successive filter clock cycles. A processor is adapted to run an algorithm to locate taps that correspond to echoes received by said digital communications receiver. The taps are selectively responsive to the processor, such that only those taps which correspond to received echoes are adjusted to update the coefficients associated therewith.
The algorithm run by the processor can comprise either a trial-and-error routine, a sweeping routine, or an off-line processing routine to identify the taps corresponding to received echoes. In an illustrated embodiment, the coefficients are updated using a moving window algorithm to enable different taps to be adjusted at different times.
The taps may be adjusted, for example, using a binary switch. Alternatively, the taps may be adjusted using an attenuator or any other suitable means.
The communications receiver may be, for example, a quadrature amplitude modulation (QAM), quadrature phase shift keyed (QPSK), or vestigial sideband (VSB) receiver.